Direct digital synthesizers (DDS) are attractive devices to digitally produce angle modulated radio broadcast signals, e.g. a carrier frequency that is modulated to produce signals combining frequency modulation (FM), coded orthogonal frequency division multiplex (COFDM), quadrature amplitude modulation (QAM), single sideband and independent sideband (SSB/ISB), frequency shift key (FSK), and phase shift key (PSK). The present invention is described for FM but many of the principles are applicable to these other types of signals.
Technology limitations of digital-to-analog (D/A) converters do not permit the generation of an FM broadcast signal containing speech and/or music broadcast information on channel (88.1 to 107.9 MHz in the United States) with the necessary degree of spurious rejection (80 dB or better). Consequently, the FM broadcast signal has in the past been generated at some lower frequency and translated to the FM broadcast band, using analog techniques.
So-called I/Q circuitry is frequently used to produce a single sideband modulated signal. The I/Q circuitry includes first and second channels each including a mixer. The mixers of the first and second channels are driven by orthogonal phases of a carrier and by orthogonal phases of a sinusoidal wave including the modulation information. Output signals of the mixers are added to eliminate one sideband and the carrier.
One way to produce a DDS FM broadcast signal is to generate the DDS FM signal at a lower frequency such as several MHz, and then heterodyne the signal up to the operating frequency. This may require a single frequency conversion or multiple frequency conversions.
Depending on the choice of an intermediate frequency (IF), a single conversion would require very good analog band pass filters. In the case of a frequency-agile system, the analog band pass filters probably require retuning if the carrier frequency is changed. Filter requirements could be relaxed through the use of a single sideband or so-called "imageless" mixer system.
A double conversion system would translate the first IF frequency to a relatively high second IF, such as 70 or 140 MHz, possibly using a SAW filter. This permits the structure for performing second conversion, to the operating frequency, to use only fixed filters.
The problems associated with these conventional digital synthesizer and frequency conversion systems are (1) digital-to-analog converter (DAC) spurious outputs, and (2) filter distortions. The DAC tradeoff is speed versus spurious output. In general, as the DAC runs faster, its spurious output increases. Conversely, as the first IF is lowered, the spurious performance improves.
There is a further tradeoff between DAC speed and complexity of the up-conversion system. The lower the IF frequency, the more complex the up-conversion system must be. Conversely, as IF frequency is raised, the design of the up-conversion system becomes easier.
The filters involved in the process can contribute frequency response and group delay anomalies. In addition, SAW filters have a "triple transit" echo distortion. Although these are linear distortions, the effects of the echo distortions on nonlinear modulation systems, such as FM, produce nonlinear distortions in the demodulated modulation functions. These distortions include nonlinear (harmonic and intermodulation) distortion, reduced stereo separation, subcarrier crosstalk, etc.
It is, accordingly, an object of the present invention to provide a new and improved angle modulated transmitter for speech and/or music broadcast information wherein the same apparatus can be used, without adjustment, for all carrier frequencies throughout standard operating bands.
Another object of the invention is to provide a new and improved angle modulated radio transmitter responsive to speech and/or music broadcast information wherein sinusoidal waves, at baseband, having an angle modulation index proportional to the speech and/or music broadcast information amplitude are directly applied, without intermediate heterodyning, to mixers responsive to the transmitter carrier frequency to produce a transmitted output wave having virtually no components other than the component at the carrier frequency, as deviated by the angle modulation of the sinusoidal waves.
An additional object of the invention is to provide a new and improved angle modulated radio transmitter responsive to speech and/or music broadcast information including a direct digital synthesizer for deriving digital signals representing sinusoidal waves at baseband.
An additional object of the invention is to provide a new and improved angle modulated radio transmitter responsive to speech and/or music broadcast information wherein the transmitter includes a direct digital synthesizer for deriving digital signals representing sinusoidal waves, wherein the sinusoidal waves have amplitudes and phase angles to compensate for imperfections in analog combining circuitry for the sinusoidal waves and the carrier frequency.
Another object of the invention is to provide a new and improved angle modulated radio transmitter responsive to speech and/or music broadcast information, wherein the transmitter has simple filtering circuitry and does not require heterodyning to an intermediate frequency.
Yet another object of the invention is to provide a new and improved angle modulated radio transmitter responsive to speech and/or music broadcast information, wherein the transmitter has a direct digital synthesizer with a relatively low capacity memory for storing signals enabling variable frequency sinusoidal waves to be derived.
A further object of the invention is to provide a new and improved angle modulated radio transmitter responsive to speech and/or music broadcast information, wherein the transmitter has a direct digital synthesizer for deriving a phase representing signal for addressing a memory which stores N bit words representing sinusoidal wave information, wherein the synthesizer responds to the N bit words to derive (N+P) bit words representing the amplitudes of different phases of the sinusoidal wave.
Yet another object is to provide a new and improved single sideband I/Q modulator that is adjusted to compensate for unwanted components while it is driving a load in response to broadcast information.